An optical fiber used for communications includes a core and cladding concentrically surrounding the core. Considering that the optical fiber may have an outer diameter of about 125 microns (10.sup.-6 m; .mu.m) and that a typical core measures about 2 to 50 .mu.m in diameter, the connection of two optical fibers so that their cores are aligned is a formidable task. In order to establish such a precise connection between optical fibers, several different connection configurations have been developed.
One known configuration is referred to as a biconic connection. An example of a biconic connection is shown and described in U.S. Pat. No. 4,787,698 to Lyons et al. This connection includes facilities for holding two plugs, each of which terminates an optical fiber and each of which has a conically-shaped endface. The optical fiber terminates in a pedestal which extends beyond an endface of the plug. Two plugs are received in opposite ends of a sleeve which is mounted in a housing. The sleeve includes opposed, conically shaped cavities for receiving the plugs and for holding them in a manner to cause the endfaces of the optical fibers to engage each other and to precisely align the optical fibers.
Another known configuration for establishing a connection between optical fibers is referred to as a ferrule connection. An example of a ferrule connection is shown and described in both U.S. Pat. No. 5,738,508 to Palmquist and U.S. Pat. No. 4,738,507 to Palmquist. The ferrule connection includes a coupler having a plug-receiving tubular portion at each end thereof. Each tubular portion is provided with a longitudinally extending slot. A sleeve which floats within the coupler is adapted to coaxially receive two plugs, each of which is adapted to terminate an optical fiber. Each plug has a passageway extending longitudinally therethrough for receiving an optical fiber and is mounted in a connector body having an alignment pin projecting radially therefrom. When the connector body is received in a tubular portion of the coupler, the alignment pin is received in the slot which extends along the tubular portion.
In both of the aforementioned connections, the endfaces of separate optical fiber terminations are joined by a coupling structure. Furthermore, to enable optimal performance, the coupling structure should join the termination endfaces so that the core and cladding are substantially contiguous and aligned. In order to join the optical fiber terminations in this manner with these coupling structures, the termination endfaces, which typically includes the fiber (core and cladding) and a surrounding termination support material, should be substantially continuously domed, or exhibit a spherical curvature. To achieve this configuration, the termination endface is typically machined by grinding and/or polishing.
If the fiber is recessed within the surrounding termination support material, then the termination endface is said to be "undercut." In this situation, after the termination endfaces have been joined by the coupling structure, reflections will arise during operation because signals must pass through an air gap when the signals travel through the coupling structure. In contrast, when the fiber "protrudes" out from the surrounding termination support material, then the fiber is susceptible to fracturing or suffering damage when joined by the coupling structure.
The amount of termination undercut or protrusion (hereafter, undercut/protrusion) can be determined manually by a visual inspection of the termination endface through a microscope equipped with interferometric optics. However, with this technique, there is no way for quantifying the undercut/protrusion with precision and repeatability. Furthermore, visual inspection requires an unacceptably long time period to reach a conclusion.
Another known technique for determining the degree of undercut/protrusion involves constructing a three-dimensional (3D) surface image model of the termination endface. After the 3D surface image model has been constructed, then the extent of undercut/protrusion is determined by visually examining the image on a display and by making estimations and calculations. An example of a commercially available apparatus for performing the foregoing methodology is called a WYKO Topological Measurement System, which is manufactured by WYKO, Inc., U.S.A. However, this technique requires sampling and measurement of numerous image planes for construction of the 3D surface image, resulting in a computationally intensive operation that is again time consuming. In fact, the 3D image model typically takes several minutes to generate.